https://itmanual.net/ios-%ec%97%85%eb%8d%b0%ec%9d%b4%ed%8a%b8-%ec%98%a4%eb%a5%98-%ed%95%b4%ea%b2%b0-%eb%b0%a9%eb%b2%95-%eb%b0%8f-%ec%95%84%ec%9d%b4%ed%8f%b0-%ec%97%85%eb%8d%b0%ec%9d%b4%ed%8a%b8-%ec%98%a4%eb%a5%98-4000/